1. Field of the Invention
This invention relates to a cylindrical cell comprising an electrode body arranged in a bottomed cylindrical outer can, the electrode body being formed by stacking a positive-electrode plate holding an positive-electrode active material and a negative-electrode plate holding a negative-electrode active material with a separator interposed to separate the positive-electrode plate and the negative-electrode plate and rolling them into a cylindrical shape with the negative-electrode plate outside, and a negative-electrode plate for use in such cylindrical cell.
2. Description of the Related Art
Cylindrical cells, such as cylindrical nickel-cadmium, nickel-hydrogen, or lithium-ion cells, comprise an electrode body arranged in a bottomed cylindrical outer can, the electrode body being formed by stacking a positive-electrode plate holding a positive-electrode active material and a negative-electrode plate holding a negative-electrode active material with a separator interposed to separate the positive-electrode plate and the negative-electrode plate and rolling them into a cylindrical shape with the negative-electrode plate outside. The negative-electrode plate for use in such cylindrical cells is produced by applying a negative-electrode active material on an electrically-conductive, porous, negative-electrode core body made from a perforated metal sheet, for example, and subjecting it to a working process such as extending by applying pressure.
There is publicly-known a prior-art cylindrical cell of this type which, in order to have an increased capacity, comprises a negative-electrode plate designed such that the parts forming the innermost and outermost circumferences of the electrode body are half as thick as the other part (see Japanese Patent Application Laid-open No. 06-267583). There is also publicly-known a cylindrical cell comprising a negative-electrode plate designed such that the two-side facing part, or part facing the positive-electrode plate on either side, with the separator between, is lower in active material density than the one-side facing parts, or parts facing the positive-electrode plate on only one side, with the separator between (see Japanese Patent Application Laid-open No. 2004-303484). There is also publicly-known a cylindrical cell comprising a positive-electrode plate with an increased thickness and a negative-electrode plate designed such that the part forming the outermost circumference of the electrode body has a reduced thickness (see Japanese Patent No. 4359099).
Generally, cylindrical cells of this type are produced by the following steps: First, an electrode body is formed by stacking a positive-electrode plate and a negative-electrode plate with a separator interposed between and rolling them into a cylindrical shape with the negative-electrode plate outside. Then, the electrode body is arranged in a bottomed cylindrical outer can, and then, a positive-electrode current-collector plate is mounted on the electrode body. Then, an electrolyte is put into the outer can through a hole in the center of the positive-electrode current-collector plate, then components including a positive-electrode lead are fitted, and last, the outer can is sealed with a lid body serving as a positive terminal of the cell.
As stated above, the electrolyte is put into the outer can through the hole in the center of the positive-electrode current-collector plate, which means that the electrolyte is put into a space in the center of the electrode body (space inside the innermost circumference of the electrode body). The electrolyte thus fills the outer can by gradually permeating the separator, positive-electrode plate and negative electrode plate, from the center to the periphery of the electrode body, where if the electrolyte does not satisfactorily permeate the electrode body, it follows that the whole electrode body fails to be uniformly activated, possibly resulting in deteriorated cycle performance and high-rate discharge performance. Thus, in manufacture of cylindrical cells, it is required to ensure that the electrolyte permeates the electrode body satisfactorily.
However, the electrode body including the negative-electrode plate designed such that the first turn forming the innermost circumference of the electrode body has an increased negative-electrode active material density, as in the cell disclosed in Japanese Patent Application Laid-open No. 2004-303494, for example, exhibits a greatly-decreased permeability to the electrolyte, resulting in increased time taken to put a required amount of the electrolyte into the outer can and let it permeate the electrode body, and thus, decreased efficiency of manufacture of the cylindrical cell.